When you say that the noise is only between 1200 and 2000 RPM it sounds like you're saying the noise only happens when you have the engine running at just about idle speed, and you probably have the clutch in also.
There's a noise EX-250s sometimes make at idle (within the conditions you've described) that tells you the hard-rubber bumpers that normally are the interface between the engine's output shaft (crankshaft) and the backside of the clutch basket have deteriorated or are completely gone. Once these hard-rubber bumpers are gone the metal "dogs" on the engine's output shaft will rattle against the metal of the clutch basket.
All this is located inside the rounded metal housing just below and slightly aft of the area you've got circled in your picture (where the clutch cable connects and the engine oil filler cap is located). So it's easy to misdiagnose the source of the noise. People often think it's coming from the Cam Chain Tensioner (CCT) which is the area you've circled on your picture.
Take a look at the information in this article, it contains a description of the problem and also a way to diagnose whether this problem is truly what's wrong with your bike's engine:
http://faq.ninja250.org/wiki/What%27..._the_clutch%3F
One way to tell for sure if the hard-rubber bumpers are coming apart is to change the oil and while you've got the crankcase empty you remove the oil-screen to see if there are chunks/bits of hard, black rubber all over it.